Observation of a bottleneck in the vibrational relaxation of liquid bromoform

We present a study on the vibrational dynamics of the C-H stretch vibration of bromoform (CHBr₃) in the pure liquid using picosecond mid-infrared pump-probe spectroscopy. Delay scans and transient spectra confirmed earlier results that relaxation occurs via an intermediate level, which causes a transient red shift of the C-H stretch frequency. We determined this red shift to be 17 cm-¹ and identified the intermediate level with the singly excited C-H bend mode. We observed that relaxation from the intermediate level is exceptionally slow and non-exponential. These observations suggest the presence of a "bottleneck" in the relaxation. The occupation of a long-living "bottleneck" level may also be responsible for the observation of a long-living absorption around 3022 cm^-1.